Get a Straight Answer

Please note!

Listed below are questions submitted by e-mail to the author of "The Great Magnet, the Earth." Some of them (marked ***) came in response to an earlier site "The Exploration of the Earth's Magnetosphere" and are also found there in the question-and-answer section.
Only some of the questions that arrive are listed, either because they keep coming up again and again--on the reversal of the Earth's magnetic field, for instance--or because the answers add extra details, which might interest other users.

From the magnetization of the sea floor we now know better (see
http://www.phy6.org/earthmag/reversal.htm). Those observations
led to "plate tectonics," suggesting that pieces of the crust indeed
move slowly, but not at all in unison--different plates move differently.
India has moved from south of the equator to where it is now (and the
collision continues, raising the Himalaya mountains). We also understand
that in the past the Earth's magnetic poles exchanged polarity now and
then, typically half a million years apart (on the Sun this happens
every 11 years or so).

The poles thus don's shift. Land masses may shift--one half of San
Francisco is sliding past the other half, for instance, with the San
Andreas fault in between. But the rate is only about one inch per year.

Reply

Dear Nalin

Ned (Edward) Benton was a researcher of geomagnetism, who studied the slow changes of the core's magnetic field. He died in the mid-1980s,
of cancer and still quite young.

The Earth's field slowly changes, all the time. As explained in the
section on Gauss, the field can be resolved into terms that depend on distance R like 1 over the 3rd, 4th, 5th etc. power of R: the dipole goes like 1/R3
and increasing powers are associated with parts of increasing
complexity.

The complex parts--e.g. those that go like 1/R7 are very small at
the surface of the Earth, but clearly they grow much faster than the
dipole as you move inwards: at a distance of half the radius of the
Earth, for instance, the dipole field is 8 times stronger, but the
1/R7
Actually, of course, you can only do so up to the boundary of the
core: after that you are in a region of electric currents, and the
description introduced by Gauss must be replaced with a different one,
which does not grow so fast.

The Earth's dipole field is declining at a rate that suggests it will
cross zero before the year 4000. Benton (and his colleague Coerte Voorhies) however knew that a certain theorem, developed by Raymond Hide, required the total unsigned flux leaving the surface of the core (counting negatives as if they had a positive sign, too) to stay the same all the time (assuming the electric conductivity is very high).

This does not allow the field to simply decay. What seems to be happening instead that magnetic flux (intensity time area, weighed over an area in a certain way) of the main north-south field is indeed weakening, but more complex parts of the field meanwhile increase. These component decay more rapidly with distance from the Earth's core, so they contribute relatively little, and the overall result is that as the field grows more complex, it also gets weaker. During reversals you will see a weaker magnetic field and perhaps 4 or more magnetic poles, but the field never vanishes.

You can now see why I was reluctant to go into details! They can be much
more complicated. But that is what physics usually involves.

Question #6-A

I need to know as much as possible about the reversal of the magnetic
field:

how it was noticed

who discovered the reversal

how long ago did it reverse

how many times did it reverse

more information about the radiation from the sun if the magnetic
field reverses.

Reply

ABOUT GEOMAGNETIC REVERSALS: This is a huge subject and I cannot do quick justice to it: look up in the index volume of the Britannica under: Geomagnetism, Plate tectonics, Reversals of the Earth's magnetic field.

HOW IT WAS NOTICED: When lava pours from a volcano, it solidifies to a black rock called basalt. Basalt is slightly magnetic, and it takes on the direction
of the surrounding magnetic field at the time it solidifies. Scientists
examined lavas for their magnetism early in this century (I believe) to see
how consistent the direction of ancient magnetic fields was with the direction
we observe now (would compasses point in the same direction?). The directions
generally agreed, but there existed reversals of directions which suggested
that there were times in the past when the poles were roughly interchanged.

No one knew what to make of it. Some suggested "polar wandering", that
the whole surface of the Earth slid around the interior like a loose shell.

WHO DISCOVERED: I don't remember. Check a book by Allan Cox, a collection
of historic articles.

But a big change happened in 1963. People noted that while rocks on Earth
were magnetized in a disordered way, the sea bottom was magnetized in long
strips. Larry Morley (whose article was regarded so speculative that journals
would not publish it) and then Matthews and Vine (who managed to publish)
suggested that molten rock was spreading out like a conveyer belt from volcanic
cracks in the middle of the ocean floor, e.g. the one in the middle of the
Atlantic (Azores islands sit on it). Or rather like 2 belts, one moving towards
Europe, one towards America, carrying on them the continental plates, so that
Europe and America gradually drift apart. As each belt comes out of the crack,
its lava solidifies to basalt, causing it to become magnetized, and when the field
reverses, its magnetization reverses too. So the bottom of the ocean records
the field like the tape of a tape recorder, containing perhaps 50 million years
of record.

HOW LONG AGO: about 700,000 years, according to the "tape recorder"

HOW MANY TIMES: Many, about half a million years apart on the average.

RADIATION FROM THE SUN: Sunlight of course is undisturbed. High-energy
protons from the Sun are usually diverted by the magnetic field. During
the reversal the field probably does not disappear, but becomes complex
and weaker, and protons can more easily reach the atmosphere, as they do now
within 1000 miles or so of the magnetic pole. On the ground it makes
no difference because the thick atmosphere shields us very well, and none
of the protons penetrates far into it.

Question #6-B

Reversal of magnetosphere

We have been studying the magnetosphere and the Van Allen radiation
belts in a high school physical science class. It has been brought to
our attention that the magnetic poles of the earth reverse on an average
of about every 500,000 years. The last change was about 700,000 years
ago, so it would appear that we are long overdue.

What are the implications of this? How significant would the
fluctuation of the magnetic field during such a change be on our
protection from solar wind?

Ricky

Reply

Dear Ricky

Only yesterday a similar question was submitted, so as a shortcut a copy of it [next item below] and its answer are attached below.

Some people worry that during magnetic reversals the Earth
would receive a higher dosage of high-energy ions and electrons
("radiation" in common terms), which might affect us and any living
creatures on Earth. This is not so. Even today, the magnetic shield
is not effective near the magnetic poles, yet the radiation received
there on the ground is only slightly higher than anywhere else. The
reason is that our main shield against such particles is not the magnetic field of the Earth but the atmosphere, equivalent to some
10 feet of concrete.

In any case, during reversal the magnetic field does not go away,
it only gets weaker and develops several more magnetic poles,
at unpredictable locations.

Question #6-C

Could you tell me when the earth's magnetic poles will change, and what
will happen when it does? Will it happen fast (seconds) or slowly?
Thank you!
Sarah

Reply

Dear Sarah

No one knows when the next field reversal will occur: in the past,
they have occurred on the average about once in 500,000 years. The change, whenever it occurs, will be gradual and the field will not drop to zero in between--doing so would mean that the magnetic energy of the Earth was somehow converted or dissipated, and all processes we know for this tend to run on scales of thousands of year, if not more.

Right now the main (dipole) field is getting weaker at a rate of about 7% per century, and if you draw a straight line through the points you find it reversing between 1000 and 2000 years from now. It might happen, though the trend may also change before then. But as explained elsewhere, even if a reversal occurs, the field does not disappear during the time of polarity change, it just gets more complex and weaker.

The polar field of the Sun seems to reverse every 11 years or so,
taking about a year or more. But the Sun's magnetism is different,
it has foci right on the surface, in sunspots.

Question #6-D

Earth's magnetic field weakening--leading to a pole shift?

I am just a tax paying citizen, interested in astronomy all of my life.
I am very interested in the physics of our Earth which I believe is
related to astronomy as it is our home and a part of this solar system.

My question is: Is the Earth"s magnetic field weakening, heading to
zero point? With this, is the base pulse frequency of the Earth
speeding up causing the magnetic fields to fluctuate so that it
interferes with the pilots navigational equipment, so that
the navigational charts have to be redrawn periodically and the
air strips renumbered? Are the magnetic poles fluctuating? My
experience is that they are. I have a quality, liquid filled compass
secured to my desk. It has been very still now for the past month but
the six weeks or so prior to that, there were consistent fluctuations
in its direction, up to as much as 2 1/2 degrees, always to the west.

My understanding is: I have seen photographs of the sun taken from
satellites, showing the sun going through major activity. Repolarizing
itself? Causing the earth to repolarize itself? Going through a natural
cycle as it has many times in the past with pole shifts? On a scale
from 1 to 10, with 1 being the weakest and 10 the strongest, 2,000 years ago
it was a 10, today it is a 1. Is it heading for a zero point when a pole shift
will occur? The closer it gets to the zero point, the more fluctuations
will occur?

Are the change in the magnetic frequencies causing at times a confusion
in migratory animals? Causing cells to mutate, changing the DNA pattern
within the cell? Causing certain strains of bacteria such as staph
infections to become resistant to our antibiotics and causing new
viruses to appear that we have never seen before, being able to survive in
a new magnetic frequency?

I believe these are very fascinating times in which we live. The science
of all of this intrigues me to no end. I have some taped interviews
of scientists and geologists relating to this subject and I read all that
I can get my hands on, on the subject also. Your straightforward comments
and answers will be most welcomed to help me to understand more, what
is taking place. Thank you so very much.

Michael

Reply

Dear Michael

Questions on reversals regularly arrive at this desk, and some others are listed above. By the way, the source of the magnetic field is not any polarization (at the Sun or Earth) but electric currents, flowing below the visible surface of the Earth or Sun. In the present collection "The Great Magnet, the Earth you will find a great deal of material on the magnetic fields of Earth and Sun and the way they probably arise.

Now to your questions.

Is the Earth's field getting weaker? Yes and no. That field is often
viewed as being a two-pole ("dipole") structure similar to that of a
small bar-magnet at the center of the Earth, inclined by about 11 degrees
to the rotation axis of the Earth, so that the magnetic poles are not the
same as the geographic ones. But the actual situation is more complicated,
and magnetic charts note the fact by mapping deviations between magnetic
north and the direction to the magnetic pole, which fit no simple pattern.

Why? Because the magnetic field is actually more complicated, and it
contains additional fields, of more complex nature. All this originates
in the Earth's core, about half the radius of the Earth. If we could go
to the surface of the core, all the complicated parts would be much
bigger. But they weaken more rapidly with distance, so at the surface
of the Earth they are already quite weak, while the "dipole" part
stands out more (in addition of actually BEING the biggest chunk of the
field).

Are you still with me?

The magnetic field of the Earth changes all the time, and yes, magnetic
charts have to be redrawn from time to time (this was first found in
1641, by an Englishman named Gellibrand). And yes, in the century
and a half since the first careful mapping of the Earth's field, the
dipole has become weaker by about 8% (the rate may have speeded up in
1970). If you draw a straight line through the points, you will find
that perhaps 1200 years from now, the line goes through zero.

Extending straight lines too far beyond the present, however, is risky
business, as noted by no less a scientific authority than Mark Twain.
In "Life on the Mississippi" Twain noted that the Mississippi river was
getting progressively shorter (mainly by floods--and by people--creating
shortcuts through bends in the river) and he wrote:

"Now, if I wanted to be one of those scientific people, and "let on"
to prove what had occured in the remote past by what had occured in
a given time in the recent past, or what will occur in the far future
by what has occured in late years, what an opportunity is here! ...
Please observe:

In the space of one hundred and seventy six years the lower Mississippi
has shortened itself two hundred and forty-two miles. That is an average
over a mile and a third per year. Therefore, any calm person, who is not
blind or idiotic, can see that in the lower Oolitic Silurian Period,
just a million years ago next November, the lower Mississippi was upward
of one million three hundred thousand miles long, and stuck out over the
Gulf of Mexico like a fishing rod. And by the same token any person can
see that seven hundred and forty years from now the lower Mississippi
will be only a mile and three quarters long, and Cairo and New Orleans
will have joined their streets together, and will be plodding comfortably
along under a single mayor... There is something fascinating about science.
One gets such wholesale returns of conjecture out of such a trifling
investment in fact."

It is not impossible that the magnetic field will go through zero
1200 years from now, but (judging by the past record of reversals) not likely.
In any case, the field is not going away: a "flux preservation theorem" suggests this is not happening (at least not on the relatively fast time scale of observed variations of the field; see here). In agreement with that theorem, one finds that while the dipole
field is getting weaker, the complicated parts are getting stronger. That's why I wrote "yes and no." During a reversal the two-pole (dipole) component of the field (which now dominates it) may go through zero, but the complex parts of the field will be relatively high, and because of them, while the overall field will be weaker, it won't vanish.

I don't know about migrating animals (they may have magnetic organs,
sort of built-in compasses), but there seem to exist no magnetic effects
on DNA, resistance to antibiotics and so on; those changes seem more
related to chemistry.

Finally, be cautious with compass readings in your house. Houses do
contain electric currents and machinery, and these may affect the readings
of a magnetic compass. On NASA's satellites the magnetic sensor usually
sits at the end of a long boom, to keep it away from interfering electric
currents in the satellite's circuits.

The text seems to be in Japanese, for on my computer it does not give anything readable, but the maps are in English. Clicking on the first will show you that the magnetic intensity around the equator varies quite a bit. but 30,000 gamma (or nanotesla, same thing) is a reasonable value.

The field has been weakening since Carl Friedrich Gauss measured it around
1836, by about 5% per century, recently accelerating to 7%/century. The total
energy of the field however is nearly constant, as shown by the late Ned Benton. This means that the field is not really weakening, only reshuffling its energy, reducing the "main dipole" (=north-south bar-magnet pattern, declining as noted by about 7% per century) and reinforcing the more complicated parts.

These tend to contribute a weaker field, because the magnetism originates in
the Earth's core, about half an Earth-radius down: all magnetic fields at the
surface are weaker than those in the core, because of the distance, but the more complicated fields decrease faster.

Whether the main dipole will reverse in about 1300 years is anyone's guess.
Geological evidence suggests it has happened in the past, but odds are against
it, because the mean frequency of such reversals in the past seems to be about
once in 500,000 years.

If you pass an electric current through water in the presence of a
magnetic field, its flow will be pushed aside. That force--it's a force on an electric current, the water is just incidental--can be used to pump water, but it is an inefficient method, because an electric current
in water wastes energy and promotes corrosion. However, in "fast breeder"
nuclear reactors, such as "Superphenix" in France, liquid sodium is
used as coolant, and is pumped by "electromagnetic pumps" based on this
principle. In such a reactor ("fast" refers to the type of neutrons used,
not to the reactor) heat is generated by fission in a rather small volume,
maybe a meter (3 ft) across, and since the heating rate is very high, it
must be removed quickly. Liquid sodium is a good coolant, since it conducts
heat very well, and it conducts electricity well, with small wastage. Unfortunately it is also tricky to handle and will burn when exposed to air. In 1990 Superphenix was shut down because of leaks
of its coolant, and it has only operated intermittedly since then. See:http://ccnr.org/news/news_briefs_12.html